A major 4 Ma palcothermal anomaly is present around the Silver Creek stockwork Mo deposit near Rico, southwestern Colorado. The anomaly extends at least 8 km outward from the deposit and was formed from heated, meteoric water circulating around an intrusion related to the deposit. The top of the 4 Ma Mo deposit is located near the center of the anomaly, more than 1 km beneath the present ground surface. Near-surface contemporaneous vein and replacement base and precious metal deposits are peripheral to the Mo system; they were the source of historic production in the Rico mining district.The thermal and chemical effects of the paleothermal anomaly are well developed in 65 Ma hornblende latite sills that extend throughout the district. They provided an essentially uniform material in which to measure the effects of water-rock interaction. Hydrothermal alteration in the latite resulted in several alteration facies. A texturally destructive facies proximal to the center of the system consists of quartz-illite-calcite and chlorite-epidote assemblages and is restricted to a vertical cylindrical zone, 3 km in diameter, directly above the stockwork Mo deposit. Peripheral to this, the latite is altered to a propylitic assemblage, with primary textures preserved. Temperature gradients in the altered latite, measured from tetrahedralsite AI in hydrothermal chlorite and the degree of partial annealing of fission tracks in apatite and zircon, are symmetrically distributed about the stockwork Mo deposit and show that temperature increased gradationally toward the heat source. The chlorite tetrahedral Al geothermometer indicates that temperatures were less than 200 degrees C in the distal part of the system and greater than 300 degrees C in the center of the paleothermal anomaly, directly over the stockwork Mo system. Whole-rock delta 18 O values of the latite decrease from near fresh-rock values of about 9.0 per mil to as low as -4.0 per mil above the Mo deposit. Mass exchange between fluid and latite is extensive above the Mo deposit, where nearly all of the Na 2 O has been removed from the rocks. The extent and distribution of thermal and chemical effects in a vertical cylindrical volume of rock above the heat source suggests that this marks the pathway for an upwelling geothermal plume, 3 km wide and at least 2 km high. A horizontal outflow plume moved laterally away from the rising plume 1.5 to 2 km above the stockwork Mo system. Water/rock ratios within the plume area are greater than 0.5 on a molar oxygen basis, but are less than 0.45 in the propylitized latite outside the plume. All mineralization in the Rico district is related to this 4 Ma meteoric-magmatic hydrothermal system. The deep stockwork Mo deposit is beneath the plume and the shallower veins and replacement deposits are at the margins of the plume.

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